(1) Field of the Invention
The invention relates to a light grid including a predetermined number of pairs of transmitters that emit light rays and receivers, which together form beam axes.
(2) Description of the Related Art
A light grid of this type is known from German Reference DE 201 04 248 A1. This known light grid comprises a transmitting unit and a receiving unit, arranged at opposite sides of a monitoring range, for monitoring a two-dimensional monitoring range. The transmitting unit comprises an arrangement of side-by-side positioned transmitters that emit transmitting light rays. The receiving unit comprises a corresponding arrangement of side-by-side positioned receivers. With a clear beam path, the receivers receive the transmitting light rays emitted by the transmitters. If an object is located in the beam path, the transmitting light rays of at least one transmitter are interrupted, so that these rays no longer reach the corresponding receiver or receivers, which causes an object detection signal to be generated.
Light grids of this type are used in particular for the protection of persons. For example, light grids of this type are used to monitor safety-critical areas around machine tools, such as presses or the like. If a foreign object enters the region monitored by the light grid, the machine tool is stopped when the object detection signal is generated in the light grid, so that there is no danger to a person working in the area of the machine tool.
Depending on where the light grid is installed, the region to be monitored may also contain objects that do not endanger persons. For example, stationary posts, machine parts or the like can be arranged in the area around the machine tool and can project into the monitoring range. A response by the light grid, such as the object detection signal, would be undesirable in cases where objects that would not endanger persons are arranged around the machine.
In order to use a light grid to protect against danger in a case of this type, reference DE 201 04 248 A1 suggests blanking out those regions within the monitoring range, which contain stationary, non-dangerous objects. Objects are then detected only in those parts of the monitoring range, which are not blanked out.
At least one transmitting adapter must be attached to the transmitting unit and one receiving adapter to the receiving unit for blanking out zones within the monitoring range. For this, the transmitting light rays from at least one transmitter are coupled via the transmitting adapter into a transmitting path that extends outside of the monitoring range. At the output of the transmitting path, bridging light rays are coupled via the receiving adapter into the receiver assigned to the transmitter, so that reference light signals are generated at its output.
The disadvantage of this arrangement is that additional sensor components are required for blanking out zones in the light grid, which components must be attached to the light grid. This leads to an undesirable increase in production costs for the light grid and, additionally, leads to higher assembly costs for attaching the transmitter and receiver adapters to the light grid.